Electronic toll collection systems conduct toll transactions electronically using RF communications between a vehicle-mounted transponder (a “tag”) and a stationary toll plaza transceiver (a “reader”). An example of an electronic toll collection system is described in U.S. Pat. No. 6,661,352 issued Dec. 9, 2003 to Tiernay et al., and owned in common with the present application, the contents of which are hereby incorporated by reference. Typical electronic toll collection systems provide for toll plazas having reader antennas located above the toll road mounted on an overhead gantry or other support structure. The overhead antennas are directional and are intended to communicate with transponders or tags passing beneath them on the toll road.
A vehicular transponder or tag is typically one of two types: a windshield transponder or a license plate transponder. The windshield transponder is mounted to the interior or exterior of the windshield and is particularly well adapted to communicate with an overhead antenna. Due to its location, the windshield transponder usually enjoys a clear RF transmission path to an overhead gantry.
For various reasons, a vehicle may be equipped with a license plate transponder instead of a windshield transponder. The license plate transponder is less well adapted for communications with an overhead gantry than the windshield transponder. The license plate transponder suffers from the dB loss associated with being mounted lower on the vehicle, i.e. further from the overhead antenna. In addition, the license plate transponder may encounter multipath problems due to reflections off of the road surface. These multipath problems can result in signal cancellation at certain mounting heights
Accordingly, it would be advantageous to provide for an improved electronic toll collection system or method of operating such a system.